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Although veins appear blue through the skin, blood is not blue. The reason why veins might seem to be blue may have to do with the level of oxygen in the blood.

Arteries transport blood that is rich in oxygen away from the heart to be used by organs and tissues in the body. The veins return deoxygenated blood to the heart.

It is a common myth that veins are blue because they carry deoxygenated blood. Blood in the human body is red regardless of how oxygen-rich it is, but the shade of red may vary.

The level or amount of oxygen in the blood determines the hue of red. As blood leaves the heart and is oxygen-rich, it is bright red.

When the blood returns to the heart, it has less oxygen. It is still red but will be darker. This darker red appears blue because of how light travels through the skin.

2. Do some animals have blue blood?

Similar to people, most animals also have red blood. However, there are a few exceptions.

Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color.

Blue and red are not the only possible colors of blood — some animals bleed green.

The skink, which is a type of lizard, has green blood due to a buildup of biliverdin. Biliverdin and bilirubin are by-products of the liver. Humans also produce these two by-products.

However, the human body sends biliverdin and bilirubin to the intestines and through the digestive system to be excreted. Skinks do not excrete biliverdin, so it builds up in their body, making the blood green.

3. How much blood is in the body?

Everyone knows that blood is a vital component of the human body. But just how much blood is in the body? The exact amount of blood in the body varies according to a person's size. The larger a person is, the more blood volume they have.

Approximately 7–8 percent of a person's total weight is blood. That means an average-sized woman has about 9 pints of blood and an average-sized man about 12 pints.

5. What are blood types?

Although everyone's blood contains the same elements, not everyone's blood is the same.

Everyone's blood contains the same elements or components. Human blood contains red cells, white cells, platelets, and plasma. Even though all blood contains those same components, not everyone's blood is alike.

There are different blood types based on the absence or presence of specific antigens and antibodies on the surface of the red blood cells. An antigen is a substance that can cause an immune system response in the body.

The two most common antigens are A and B. For example, people with the blood type A have an A-antigen on their red blood cells, and people with type B blood have a B-antigen. Some people have both.

People with O blood type do not have either A or B antigens on the red blood cells. Type O is the most common blood type in the world.

Another antigen is a protein called the Rhesus (Rh) factor. People with this protein are considered Rh-positive. If the blood lacks the protein, they are Rh-negative.

It is more common to have Rh-positive blood than Rh-negative. If a person who is Rh-negative needs a transfusion, they should not receive Rh-positive blood.

Anyone of any ethnic group can have any blood type, but there are some ethnic and racial trends.

For example, having blood type B is relatively common for people who are Asian or of Asian descent.

Although A and B are the most common antigens, there are many other less common antigens that create rare blood types.

Conditions that affect the red blood cells, such as sickle cell disease.

If a person requires a blood transfusion, it is essential that the blood type they are given is a compatible type. If a person receives an incompatible blood type, their immune system may reject it. This can be life-threatening.

Most people can receive type O blood safely, regardless of their blood type. This is why most blood donation clinics are in need of type O blood, as it can be used to help many people.

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